|
The Baeyer-Villiger oxidation (also called Baeyer-Villiger rearrangement) is an organic reaction that forms an ester from a ketone or a lactone from a cyclic ketone.〔 Peroxyacids or peroxides are used as the oxidant.〔 The reaction is named after Adolf Baeyer and Victor Villiger who first reported the reaction in 1899.〔 ==Reaction mechanism== In the first step of the reaction mechanism, the peroxyacid protonates the oxygen of the carbonyl group. This makes the carbonyl group more susceptible to attack by the peroxyacid.〔 In the next step of the reaction mechanism, the peroxyacid attacks the carbon of the carbonyl group forming what is known as the Criegee intermediate.〔 Through a concerted mechanism, one of the substituents on the ketone migrates to the oxygen of the peroxide group while a carboxylic acid leaves.〔 This migration step is thought to be the rate determining step. Finally, deprotonation of the oxygen of the carbonyl group produces the ester.〔 The products of the Baeyer-Villiger oxidation are believed to be controlled through both primary and secondary stereoelectronic effects.〔 The primary stereoelectronic effect in the Baeyer-Villiger oxidation refers to the necessity of the oxygen-oxygen bond in the peroxide group to be antiperiplanar to the group that migrates. This orientation facilitates optimum overlap of the 𝛔 orbital of the migrating group to the 𝛔 * orbital of the peroxide group.〔 The secondary stereoelectronic effect refers to the necessity of the lone pair on the oxygen of the hydroxyl group to be antiperiplanar to the migrating group.〔 This allows for optimum overlap of the oxygen nonbonding orbital with the 𝛔 * orbital of the migrating group.〔 This migration step is also (at least in silico) assisted by two or three peroxyacid units enabling the hydroxyl proton to shuttle to its new position.〔''The Role of Hydrogen Bonds in Baeyer-Villiger Reactions'' Shinichi Yamabe and Shoko Yamazaki J. Org. Chem.; 2007; 72(8) pp 3031–41; (Article) 〕 The migratory ability is ranked tertiary ≻ secondary ≻ phenyl ≻ primary. Allylic groups also migrate better than primary groups but not as good as secondary groups.〔(【引用サイトリンク】first1=Andrew G. )〕 If there is an electron withdrawing group on the substituent, then it decreases the rate of migration. There are two explanations for this trend in migration ability. One explanation relies on the carbocation resonance structure of the Criegee intermediate.〔 Keeping this structure in mind, it makes sense that the substituent that can maintain positive charge the best would be most likely to migrate.〔 Tertiary groups are more stable carbocations than secondary groups, and secondary groups are more stable than primary. Therefore, the tertiary ≻ secondary ≻ primary trend is observed. Another explanation uses stereoelectronic effects and steric bulk to explain the trend. As mentioned, the substituent that is antiperiplanar to the peroxide group in the transition state will be the group that migrates.〔 This transition state has a gauche interaction between the peroxyacid and the non-migrating substituent.〔 If the bulkier group is placed antiperiplanar to the peroxide group, the gauche interaction between the substituent on the forming ester and the carbonyl group of the peroxyacid will be reduced.〔 Thus, it is the bulkier group that ends up antiperiplanar to the peroxide group making it the group that migrates.〔 This explains the trend of tertiary ≻ secondary ≻ primary because tertiary groups are generally bulkier than secondary and primary groups. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Baeyer–Villiger oxidation」の詳細全文を読む スポンサード リンク
|